This is trivial to do in Fiddler today. If you only want Firefox, you could just manually point Firefox's proxy settings at Fiddler and not Fiddler set to be the system proxy.

Alternatively, you can use a script, like this sample in the Fiddler book:

Hide Traffic based on Process Name

The following script creates a ShowOnly submenu on the Rules menu that allows you to easily display captured traffic from only a single executable:

RulesString("ShowOnly", true)
BindPref("fiddlerscript.rules.ProcessFilter")
RulesStringValue(0,"Chrome", "chrome")
RulesStringValue(1,"FireFox", "firefox")
RulesStringValue(2,"IE", "iexplore")
RulesStringValue(3,"&Custom...", "%CUSTOM%")
public static var sOnlyProc: String = null;

//Add the following to the OnPeekAtRequestHeaders function:

if (!String.IsNullOrEmpty(sOnlyProc)) { var s = oSession.LocalProcess;
   if (!s.StartsWith(sOnlyProc, StringComparison.OrdinalIgnoreCase)) {
     // Or use oSession.Ignore() instead of ui-hide...
     oSession["ui-hide"] = "Rules>ShowOnly";
   }
}